1. Field of the Invention
The present invention relates to a ranging method in an access network system for an optical line terminal and a plurality of optical network units to communicate using a code division multiplexing (CDM) method, for adjusting the receive timing of a signal corresponding to delay time based on the difference of distance from the optical line terminal to each optical network unit.
2. Description of Related Art
As a typical example of a communication system between an optical line terminal and a plurality of optical network units, an optical access network system constructed by connecting the optical line terminal and a plurality of optical network units via PON (Passive Optical Network) is receiving attention. The optical line terminal is an equipment installed at a service provider side, and may be called a “center node” or a “service node”. The optical network unit is an equipment installed at a subscriber (may also be called “user”) side, and may be called an “edge node”.
PON is a network for connecting an optical splitter in the middle of an optical fiber transmission line so as to split one optical fiber transmission line into a plurality of optical fiber transmission lines, and connecting a plurality of edge nodes in a star shape with this optical splitter at the center (e.g. see Hiroya Ohnishi et al: “Ethernet PON System”, Fujikura Giho No. 102, April 2002, pp. 18-21, and Yoshio Miyamori et al: “PON Based All Fiber-Optic Access System”, Hitachi Review, Vol. 47, NO. 2, 1998, pp. 63-66). By using PON for a network connecting the center node and edge nodes, the optical fiber transmission line between the center node and the optical splitter can be shared by a plurality of edge nodes, and equipment cost can be suppressed.
Hereafter communication between the center node and the N number of edge nodes may be called “1 to N communication” (here N is 2 or higher integer).
An example of a conventional optical access network system using PON involves identifying a transmission signal from each user by controlling the time slot of the TDM (Time Division Multiplexing) signal using the TDM method (e.g. see Japanese Patent Application Laid-Open No. H10-93607 and Japanese Patent Application Laid-Open No. H10-508443 (corresponding to WO 96/13915)). According to this example, a method of adjusting a signal receive timing corresponding to the delay time based on the difference of distance from the center node to each edge node (hereafter may be called “ranging method”) has already been known. In other words, in the 1 to N communication using the TDM method, the ranging method has already been disclosed.
On the other hand, if the CDM method could be used for an optical access network system using PON, various advantages, inherent to the CDM method, could be utilized.
The CDM method is a communication method of encoding and transmitting transmission signals and decoding them at the receive side using the same codes used for encoding at the transmission side. The communication method based on CDM has the following advantages. First, the communication method based on CDM is a method in which the transmission side and the receive side use the same codes as keys (codes which are set in the encoder and decoder may be called keys), so security during communication is high. Also according to CDM, it is possible to multiplex a plurality of transmission signals at the same time, so large capacity data communication is possible while conserving communication resources, such as wavelengths and time slots.
This invention is a technology which can be applied not only to an optical access network system which uses a PON type communication network, but can be widely applied to an access network which executes all the operations based on electric signals alone, without involving optical signals. However, in the following description, the optical access system which uses PON, in particular, may be described for convenience. In this case, the optical line terminal, that is a center node, may be called “OLT (Optical Line Terminal)”, and an optical network unit, that is an edge node, may be called an “ONU (Optical Network Unit)”.
As described above, if the CDM method could be used for 1 to N communication using PON, superb features of the CDM method could be utilized. When the 1 to N communication system is constructed by the CDM method, ranging, to adjust the delay time based on the respective difference of distance from the center node to each node, must be performed. However, in the 1 to N communication system based on CDM, a method for performing ranging has not been established.
With the foregoing in view, it is an object of the present invention to provide a ranging method in the case of using the CDM method for the 1 to N communication system.